I'll start the ball rolling with Kevin Spacey as a Romulan Tal Shiar agent, Ian McKellen as a member of Vulcan High Command, Liam Neeson as a traveling El-Aurian, Alan Arkin and Dannny DeVito as a Ferengi Diplomats, and Ron Perlman as a Klingon.

Klingon: Christopher Judge (Teal'c is like an honorary Klingon! Judge could play a passable vulcan, if he wasn't so HUGELY MUSCULAR), Adam Baldwin, Samual L Jackson, Clancey Brown, Tim Curry,Tony Amendola (Bra'tac), Jolene Blalock could have been a good Klingon

Klingon: Christopher Judge (Teal'c is like an honorary Klingon! Judge could play a passable vulcan, if he wasn't so HUGELY MUSCULAR), Adam Baldwin, Samual L Jackson, Clancey Brown, Tim Curry,Tony Amendola (Bra'tac), Jolene Blalock could have been a good Klingon

Also, Martin Landauwould have made a great TOS Vulcan. I want to say I once heard he tried out for Spock, when they were doing the casting, which was a coincidence, since Nimoy replaced Landau on Mission Impossible.